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1. Technical Field
This disclosure relates to the removal of peroxides from a naphtha stream or supply that has been exposed to oxygen. When exposed to oxygen, naphtha compounds can form peroxides which readily decompose to free radicals. This disclosure provides a reactor and a method for the catalytic conversion of such peroxides to useful hydrocarbons and oxygen which can be used to replace a conventional oxygen stripper column and conventional oxygen stripping method.
2. Description of the Related Art
Naphtha (C6-C10 hydrocarbons) is generated from the distillation of petroleum as well as coal, tar and shale oil and is a primary constituent of gasoline. Prior to being incorporated into a gasoline formulation, naphtha is typically hydrotreated or hydrodesulfurized.
Hydrotreating or hydrodesulfurization (HDS) is a common process to remove contaminates such as sulfur, hydrogen, condensed ring aromatics and/or metals in a catalytic process. However, prior to passing naphtha through a hydrotreating unit, naphtha often comes into contact with oxygen, either in storage or during transit. The oxygen reacts with naphtha to form peroxides, which readily decompose into free radicals. Once decomposed, the free radicals initiate the formation of oligomers (gums), which can result in fouling of the hydrotreating process unit.
Currently, such peroxides can be removed from a naphtha stream using a reboiled oxygen stripper column. The bottoms temperature of a conventional reboiled oxygen stripper column must be maintained at or above 176° C. (350° F.) to insure complete thermal decomposition of the peroxides. Thus, these conventional oxygen stripper columns have substantial energy consumption and therefore high operating costs. For example, a 28,750 BPSD oxygen stripper column requires approximately over $1.5 million in high-pressure steam per year to operate. Further, conventional oxygen stripper columns are relatively wide and therefore expensive to construct and consume a substantial footprint. For example, a typical oxygen stripper column and related equipment are very costly to construct.
As a result, some refiners bypass the oxygen stripping process altogether thereby adversely affecting the downstream naphtha hydrotreating unit. Specifically, some refiners consider it to be less expensive to incur the additional downtime for purposes of unfouling the naphtha hydrotreating unit rather than investing in an oxygen stripper column and incurring the additional capital and operating costs.
Therefore, there is a substantial need for an improved oxygen stripping process and equipment for naphtha streams that may be inexpensively incorporated into a refining process upstream of a naphtha hydrotreating unit.
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OF THE DISCLOSURE
In satisfaction of the aforenoted need, an improved oxygen stripper process and reactor is provided for efficiently decomposing peroxides found in naphtha streams that have been exposed to air or oxygen. As noted above, conventional processes rely upon the following thermal decomposition:
which requires a reaction temperature of at least 176° C. (350° F.).
To reduce the energy consumption required by the above endothermic reaction, disclosed herein is a catalytic reactive oxygen stripping process as follows:
which requires a reaction temperature below 176° C. (350° F.), thereby reducing energy consumption.
In the disclosed process, a reactor is provided. The “exposed” naphtha stream that comprises as least some peroxides is delivered to one end of the reactor column. The reactor column houses a catalyst bed disposed between the opposing ends of the column. The naphtha stream needs to be heated, but only to a temperature of less than 350° F. in contrast to the higher utility requirements of conventional processes. As the naphtha stream passes through the catalyst bed, the catalytic conversion of the peroxides to hydrocarbons and oxygen takes place and the stripped naphtha and resulting oxygen is removed from the reactor.
Numerous catalysts will work in the oxygen stripping process. Essentially, numerous aluminum based catalysts with an additional metal or metal atoms such as iron, titanium, platinum, copper, nickel and molybdenum can be used. Similarly, zeolite catalysts with any of the above metal atoms can be utilized. Iron-zeolite and iron-alumina catalysts may be preferable because of their low cost.
In an embodiment, the exposed naphtha feed stream that is delivered to the reactor is heated in a first heat exchanger by the stripped naphtha stream taken off from the reactor and then the exposed naphtha feed is heated in a second heat exchanger or feed heater. The second heat exchanger may be driven by steam, electricity, natural gas or other convenient utility source. Again, the naphtha feed stream does not need to be heated to a conventional oxygen stripping process temperature; temperatures of less than 177° C. (350° F.) are intended to be employed. While temperatures anywhere in the range of from about 90° C. to less than 177° C. (˜194° F. to less than 350° F.) can be employed, naphtha feed stream temperatures in the range of from about 90° C. to about 163° C. (˜194° F.-˜325° F.) will be effective, more preferably in the range of from about 90° C. to about 149° C. (˜194° F.-˜300° F.) Reaction temperatures below 93° C. (200° F.) and as low as 90° C. (˜194° F.) are anticipated.
In one embodiment the reactor has a single outlet and the stripped naphtha and oxygen are passed to a receiver where the oxygen, other gases and any water present are removed from the stripped naphtha stream.
In another embodiment, as the naphtha stream passes through the catalyst bed, a counter-current stream of gas passes through the catalyst bed which helps to entrain the released oxygen produced by the oxygen stripping reaction described above. Thus, the exposed naphtha stream passes through the reactor in one direction and a gas stream passes through the reactor and catalyst bed in an opposite direction to remove the produced oxygen and, perhaps water vapor, if present.
In an embodiment, the gas stream is nitrogen which relatively inert and provides a convenient means for removing the produced oxygen. Preferably, the nitrogen stream is heated before it is injected into the reactor thereby avoiding any quenching of the catalytic oxygen stripping reaction taking place in the catalyst bed.
In a refinement of this concept, the gas stream that is removed from the reactor is cooled thereby allowing any water and naphtha vapor contained in the gas stream to condense. Preferably, any condensed naphtha is then returned to this process and reintroduced into the column with the exposed naphtha feed.
In another refinement, the exposed naphtha feed and, preferably any condensed naphtha from the gas stream, is passed through a receiver to remove or “knock out” any water prior to delivering the naphtha stream to the reactor.
Further, in addition to the catalyst bed, the reactor may include a plurality of contacting trays disposed below the bed. Typically, the need for contacting trays is reduced to fifteen or less.
As an alternative, hydrogen can be employed for the gas stream instead of nitrogen. Combinations of hydrogen and nitrogen may also be employed. On one hand, hydrogen is advantageous because it helps reduce the presence of oligomer or gummy reaction products. On the other hand, nitrogen reduces the explosivity potential of the process. It hydrogen is used as the feed gas stream to the reactor, nitrogen may be injected after the gas stream is taken off of the reactor and cooled to the keep the receiver, if utilized, well within the acceptable gas explosivity limit.
Because exposed naphtha will typically include some water as well as peroxides, it is preferred that the exposed naphtha feed stream be passed through a receiver to remove any free water prior to introduction into the reactor. Thus, the receiver provides a convenient place to combine any condensed naphtha vapor from the gas stream with the exposed naphtha feed.
Other advantages and features will be apparent from the following detailed description when read in conjunction with the attached drawings.
BRIEF DESCRIPTION OF THE DRAWING